Rapid breathing could indicate a number of things. It could be stress from traffic in your home, or from a harassing fish. New fish will breathe rapidly from the shock of being transported. You should also test your water for high ammonia, nitrate, pH, and nitrite levels. These can be toxic to fish and cause stress. Also check for parasites or disease. If you suspect your fish is ill, check out our guide on how to spot fish illnesses.